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arXiv · 2609.26998

N/O-enhanced chemical enrichment by massive stars: the interplay of optically-thick winds and rotation

Boyuan Liu·Michela Mapelli·Lumen Boco·Xihan Ji
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Abstract

Recent JWST observations have revealed a sample of high-$z$ galaxies with highly-enhanced N/O abundance ratios, $\log\rm (N/O)\gtrsim -1.1$. Strong optically-thick winds of rotating massive stars going through the Wolf-Rayet (WR) phase provide a viable explanation for such N/O enhancements, supported by the direct spectroscopic WR signatures discovered in a few galaxies. Such strong winds are also needed to reproduce the single WR stars in the Small Magellanic Cloud (SMC). Here, we compute the metal yields of massive ($\sim 20-100\ \rm M_\odot$), metal-poor stars using two sets of \textsc{mesa} single stellar evolution models with and without optically-thick winds in a dense grid of metallicities ($Z_\star\sim 0.001-0.0045$) and initial spins ($ω\equivΩ/Ω_{\rm crit}\sim 0-0.7$). Considering an initial spin distribution based on observations of O-type stars in the SMC, we find that both sets produce strong N/O enhancements up to $\rm \log(N/O)\sim 0.1$ within a few Myr after a starburst. The models with optically-thick winds undergo a rapid transition from high N/O to the `normal' state of $\rm \log(N/O)\sim -1.5$ at $t\sim 3-4$~Myr driven by C/O-rich winds, while the transition is slower and weaker without optically-thick winds. The optically-thick-wind scenario can reproduce the nebular C, N, O, and He abundance patterns of three representative high-$z$ galaxies showing direct WR signatures: RXCJ2248-ID, Sunburst Arc, and MARTA 4327, with stellar metallicities and ages similar to those inferred from SED-fitting. This can be interpreted as a coherent evolution sequence captured before ($t\lesssim 3$~Myr), during ($t\sim 3-4$~Myr), and after ($t\gtrsim 5$~Myr) the transition. In contrast, the optically-thin-wind-only scenario ejects much less C and O via winds and cannot reproduce Sunburst Arc and MARTA 4327. The yields are available at https://zenodo.org/records/22897057.

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Boyuan Liu, Michela Mapelli, Lumen Boco, Xihan Ji. 2026-09-22. N/O-enhanced chemical enrichment by massive stars: the interplay of optically-thick winds and rotation. https://arxiv.org/abs/2609.26998

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